Public Health England warned asthma sufferers, adults and children with lung problems, and adults with heart problems not to exercise outdoors.

Meanwhile people suffering symptoms of pollution - including sore eyes, coughs and sore throats - were told to cut down on the amount of time spent outside.

The London Ambulance Service said it saw a 14 per cent increase in emergency calls for patients with breathing problems.

The smog from the Royal Greenwich Observatory on Wednesday morning

The pollution was caused by light south-easterly winds, the continental air flow and dust blown up from the Sahara desert.

A department for environment, food and rural affairs (Defra) spokeswoman said: "The high level of air pollution this week is due to a combination of local emissions, light winds, pollution from the continent and dust blown over from the Sahara.

"We want to keep improving air quality and have introduced a new five-day forecast service in addition to investing heavily in local and transport initiatives to tackle this issue head-on."

The smog descends on St Mary Cray, Orpington, on Thursday morning

This comes after people found their cars covered in a light coating of red dust last weekend.

The Met Office said a large amount of sand and dust had been swept up by storm winds in the Sahara Desert.

The airborne particles of dust were blown north to the UK where they combined with warm air and were deposited during showers.